1. Field
This present disclosure relates to a system and a method for enabling sampling of a marking material image on a image bearing surface in response to a plurality of illuminators in an image printing system.
2. Description of Related Art
Optical sensors are commonly used in a variety of printing related applications. For instance, such optical sensors are often used to measure toner density on a image bearing surface (e.g. on photoreceptors, on intermediate belts, and on documents) in a printer system. Typically, sensors are designed to sample a response of a test patch on a image bearing surface to the incident light from one or more illuminators. Most of these devices make use of constant or steady illumination throughout the sampling process. In some cases, it is desirable to illuminate the test patch of interest with more than one wavelength of illumination (e.g., with three light emitting diodes (LEDs), such as red, green, and blue LEDs), or with more than one subset of wavelengths since illuminators have spectral content at a range of wavelengths. This is especially important in applications such as xerography where the different color toners respond in different ways depending on the wavelength of the illuminator.
There are two standard approaches for sampling the response of a test patch to multiple wavelengths of illumination. In the first approach, as shown in FIG. 1, a system 100 includes multiple illuminators 102 and 104, and a single sensor 106. The multiple illuminators 102 and 104 are configured to emit a light beam in a serial fashion (one at a time—i.e., sequential or alternating) at a test patch 108 on a image bearing surface 110 in order to isolate the response of the test patch to each illuminator 102 or 104 individually. The multiple illuminators 102 and 104 are sampled individually with the single sensor 106, where each illuminator 102 or 104 is pulsed on for a duration and the resultant reflectance is collected by the single sensor 106. This first approach applies to diffuse mode as well as specular mode measurements. In this first approach, because the responses are sampled in a serial fashion (one after the other), a sensing system requires a time equal to N·T to complete the sampling, where N is the number of different illuminators being used and T is the amount of time for sampling each individual illuminator, assuming the same amount of time for each illuminator. For example, when this first approach is applied to a LED spectrophotometer sensing system used in Xerox® systems, where eight different LEDs are pulsed individually, this sensing system requires a time equal to 8·T to sample each test patch of interest. Unfortunately, this type of sequential sampling requires more time to complete than sampling with a single illuminator (N*T versus just T where N is the number of different illuminators and T is the time required to sample each illuminator). In addition, since in many applications the image bearing surface 110 is moving past the sensor 106 at print process speed throughout the sampling interval, the test patches 108 must be sufficiently large to allow for sampling over this entire period of time (8T). For patches measured on customer documents, this will also require larger amounts of wasted documents for sensing.
In such applications, it would be highly desirable to speed up the time required for sampling a given test patch. In particular, the amount of time required to sample the response of the test patch to the required set of illuminators impacts the cyclic efficiency of the print engine (e.g., how long the print engine spends making customer documents versus the total amount of time the print engine is cycled-up and running) and the amount of customer media required for the sampling (e.g., the sensing systems like the Xerox® LED spectrophotometer take the measurements on a document and so must use customer media in their sampling). Thus, reducing the amount of time required to perform the sampling and/or reducing the size of the required test patches would be highly desirable in many printing applications.
A second approach, as shown in FIG. 2, for sampling the response of a test patch 208 to multiple illuminators 202 and 204 is to design sensor hardware such that all of the illuminators 202 and 204 can be tested at once. This second approach would enable sampling the response of the test patch 208 on a image bearing surface 210 to multiple illuminators 202 and 204 simultaneously in a single sampling instant, T.
This second approach provides an option to the one-at-a-time method disclosed in the first approach that may result in improvements in machine availability and document usage. However, there are several disadvantages associated with this second approach. It typically requires a specially designed hardware, such as a separate optical path (special lenses, wavelength specific optical filters 215 and 216, and optical sensors 206 and 207) and a separate analog-to-digital (A/D) converter for each desired illumination source 202 or 204. Because of these disadvantages, the second approach can result in more complex and costly sensing systems. In addition, because of the need to split the light (e.g., using a beam splitter 213) and use optical filtering (e.g., using wavelength specific optical filters 215 and 216) to select the appropriate frequencies of interest, this second approach suffers from a higher degree of loss in illumination. Thus, in this second approach, either stronger illuminators are required or the overall signal-to-noise ratio will likely suffer. Another disadvantage in the second approach is that often the received signal is a function of document orientation so that a receiver design should attempt to collect light from a circularly symmetric geometry. The second or multiple optical path approach to simultaneous illuminator sampling may not allow for this geometric design constraint to be satisfied. Thus, both the first and second sampling techniques, each have some disadvantages.
The present disclosure proposes a system that enables the sampling of the response of the test patch on the image bearing surface to each of the illuminators simultaneously, without requiring a separate optical path for each illuminator or specialized optical components to separate the frequencies of interest, and enabling a circularly symmetric receiver optical path. In other words, a single optical path and a single wideband optical detector can be used to achieve significant reductions in the required sampling time and/or sizes of the test patches.